The neurology of sign language

Forms of sign language have developed in a number of countries. American Sign Language, which originated from French signing, has been most extensively researched. As sign language is based on gestures executed in space and perceived visually it might be thought that it would mainly be a function of the right cerebral hemisphere when this is the non-dominant one. A number of studies are reviewed showing that sign language is a language in its own right and therefore, as with spoken language, its primary site of organization is in the dominant hemisphere. This does not mean that there is not a significant contribution from the other hemisphere with an interplay between the two. Each research project usually contributes some facet of knowledge apart from the main conclusions. These included the importance of distinguishing signs from gestures, the localization of different types of signing within the left dominant cerebral hemisphere, the fact that lesions of the right non-dominant hemisphere, although not causing a loss of signing will result in dyspraxia, and that aphasic symptoms of signing and speech are not modality dependant but reflected a disruption of language processes common to all languages. Examples are given of discoveries made by the use of the newer neuroradiological techniques such as functional magnetic resonance imaging and positron emission tomography, and no doubt these will lead to further advances in knowledge. The use of sign language in the treatment of patients with verbal aphasia is considered, especially of children with the Landau-Kleffner syndrome, but therapy of this kind can be used in children with delayed language development, and in other types of acquired aphasia at any age. Other methods of treatment than signing, such as cochlear implants may be increasingly used in the future, but it seems likely that sign language will continue to be a dominant feature in the deaf culture.